Elbow and horizontal configurations in a loop reactor

ABSTRACT

Loop reactors for olefin polymerization and processes utilizing such loop reactors are described herein. In one or more embodiments, the loop reactor generally includes a plurality of vertical sections; a plurality of elbow sections connecting the vertical sections to either a horizontal section having a horizontal length (L H ) or another elbow section, at least one elbow section having an internal diameter (d), a radius (R c ) of an inner curvature and a chord length (W) and wherein the horizontal length (L H ) is from 0 feet to 3 feet, the chord length (W) is 250 inches or less and a ratio (R c /d) of the radius (R c ) of the inner curvature to the internal diameter (d) of the at least one elbow section is maintained from 2 to 4; and at least one loop reaction zone configured to polymerize an olefin monomer in the presence of a liquid diluent into a slurry comprising particles of a polyolefin polymer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. patent application Ser. No.14/283,717 filed May 21, 2014 entitled “Elbow and Horizontalconfigurations in a Loop Reactor”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a reactor system for olefinpolymerization, and more particularly to a reactor system for optimizingthe production of polyolefin polymers in a high-efficiency loop reactor.

2. Related Art

This section introduces information from the art that may be related toor provide context for some aspects of the techniques described herein,claimed below, or both. This information is background facilitating abetter understanding of that which is disclosed herein. This is adiscussion of “related” art. That such art is related in no way impliesthat it is also “prior” art. The related art may or may not be priorart. The discussion is to be read in this light, and not as admissionsof prior art.

As chemical and petrochemical technologies have advanced, the productsof these technologies have become increasingly prevalent in society. Inparticular, as techniques for bonding simple molecular building blocksinto longer chains (or polymers) have advanced, the polymer products,typically in the form of various plastics, have been increasinglyincorporated into everyday items. Polyolefin polymers such aspolyethylene, polypropylene, and their copolymers, are used for piping,retail and pharmaceutical packaging, food and beverage packaging,plastic bags, toys, carpeting, various industrial products, automobilecomponents, appliances and other household items, and so forth.

One benefit of producing polyolefin is that it is generally non-reactivewith goods or products with which it is in contact. This allowspolyolefin products to be used in residential, commercial, andindustrial contexts, including food and beverage storage andtransportation, consumer electronics, agriculture, shipping, andvehicular construction. The wide variety of residential, commercial andindustrial uses for polyolefins has translated into a substantial demandfor raw polyolefin which can be extruded, injected, blown or otherwiseformed into a final consumable product or component.

To satisfy this demand, various processes exist by which olefins may bepolymerized to form polyolefins. These processes may be performed at ornear petrochemical facilities, which provide ready access to theshort-chain olefin molecules (monomers and co-monomers), such asethylene, propylene, butene, pentene, hexene, octene, decene, and otherbuilding blocks of the much longer polyolefin polymers. These monomersand co-monomers may be polymerized in processes comprising aliquid-phase polymerization reactor, gas-phase polymerization reactor,or combinations thereof. As polymer chains develop during polymerizationin the reactor, solid particles known as “fluff”, “flake” or “powder”are produced in the reactor.

It was soon discovered that a more efficient way to produce such solidparticles of polymers was to carry out the polymerization process undercontinuous slurry conditions in a pipe loop reactor with the productbeing taken off from a number of settling legs attached to the bottomhorizontal portions of the pipe loop reactor. Multiple settling legs areinstalled and operated on a batch principle to recover solid polymerproducts. This technique has enjoyed international success with billionsof pounds of ethylene polymers being so produced annually. With thissuccess has come the desirability of building large reactors as opposedto a large number of small reactors for a given plant capacity.

However, the use of multiple settling legs presents at least twoproblems. First, it imposes a “batch” product recovery technique onto abasic process that requires continuous circulation of a slurry within aloop reaction zone. Thus, problems arise when a settling leg reaches apre-determined stage where it “dumps” or “discharges” accumulatedpolymer slurry. Each time a single settling leg operates to recoverslurry from the loop reactor, it would cause an interference with theflow of the slurry circulating upstream within the loop reactor and theoperation of a reactant recovery system connected downstream. Also,valve mechanisms connecting the settling legs to the loop reactorupstream and the reactant recovery system downstream have to be large intheir diameters, thus requiring frequent, seal-off of these valves.There is significant difficulty in maintaining a tight seal. Settlinglegs may also plug from time to time. Thus, frequent reactor down timefor scheduled maintenance is unavoidable, thereby reducing productionefficiency and incurring high production cost.

Secondly, as loop reactors have gotten larger, the use of multiplesettling legs have inevitably presented many logistic problems. In aloop reactor, if a pipe diameter is doubled, the volume of the loopreactor goes up four-fold. However, because of the valve mechanismsinvolved, the size of the settling legs cannot easily be increasedfurther. Hence, a larger number of settling legs become necessary tomeet the large footprint configuration, which in turn begins to exceedthe physical space of the land available for setting up theselarge-scale, high production capacity loop reactors.

In spite of these limitations, settling legs have continued to beemployed where olefin polymers are formed as slurry in a liquid diluent.This is because, unlike bulk slurry polymerizations (i.e. no inertdiluent) where solids concentrations of better than 60 percent areroutinely obtained, olefin polymer slurries in a diluent are generallylimited to less than 50 percent solids. Hence settling legs have beenbelieved to be necessary to give a final slurry product having a greatersolids concentration. This is because, as the name implies, settlingoccurs in the legs, and thus the concentration of the solid particlesfinally recovered from the slurry settled within the settling legs arehigher. Another factor affecting maximum solid concentration withinthese loop reactors is circulation velocity, with a higher velocity fora given reactor diameter allowing for higher concentration of the solidsproduced, since a limiting factor in a circulating loop operation isfouling due to polymer build up in the walls of the loop reactor.Without meaning to be bound by any particular theory, such fouling maybe due to operation at solid particle concentrations above a level wherethe solids can remain suspended without settling within the reactor(i.e., saltation).

One way to improve production efficiency compared to the use of multiplesettling legs is to employ a continuous product take-off line or othersimilar mechanism to continuously withdraw slurry products from the loopreactor. In addition, there is a need to reduce the footprint of a loopreactor that takes up less land space (at least in the horizontaldimension) and saves production cost, while still maintaining highproduction capacity and efficiency. For example, a single continuousproduct take-off line can replace multiple settling legs, resulting insimilar productivity while reducing reactor footprint and simplifyingreactor control mechanisms.

SUMMARY

Various embodiments of the present invention include a loop reactor forolefin polymerization. In one or more embodiments, the loop reactorgenerally includes a plurality of vertical sections; a plurality ofelbow sections connecting the vertical sections to either a horizontalsection having a horizontal length (L_(H)) or another elbow section, atleast one elbow section having an internal diameter (d), a radius(R_(c)) of an inner curvature and a chord length (W) and wherein thehorizontal length (L_(H)) is from 0 feet to 3 feet, the chord length (W)is 250 inches or less and a ratio (R_(c)/d) of the radius (R_(c)) of theinner curvature to the internal diameter (d) of the at least one elbowsection is maintained from 2 to 4; and at least one loop reaction zoneconfigured to polymerize an olefin monomer in the presence of a liquiddiluent into a slurry comprising particles of a polyolefin polymer.

One or more embodiments include the loop reactor of the precedingparagraph, wherein the horizontal length (L₁₁) is from 0.5 feet to 2.0feet.

One or more embodiments include the loop reactor of any precedingparagraph, wherein the at least one elbow section connects a verticalsection having a vertical length (L_(V)) to the horizontal section andwhere the vertical length (LV) is at least 60 times longer than thehorizontal length (L_(H)).

One or more embodiments include the loop reactor of any precedingparagraph, wherein the at least one elbow section connects a verticalsection to another elbow section.

One or more embodiments include the loop reactor of any precedingparagraph further including one or more settling leg assembliesextending from the horizontal section, one or more continuous take off(CTO) assemblies extending from at least one of the elbow sections orthe horizontal section or a combination thereof.

One or more embodiments include the loop reactor of any precedingparagraph, wherein the at least one elbow section includes a height (H)and wherein the radius (R_(c)) of the inner curvature, measured asR_(c)=H/2+W²/8 H, is 72 inches or less.

One or more embodiments include the loop reactor of any precedingparagraph, wherein the circulation velocity (V) of the slurry within theat least one loop reaction zone is maintained at 9 meters per second orhigher.

One or more embodiments include the loop reactor of any precedingparagraph, wherein a Reynolds number of the slurry within the at leastone elbow section is maintained at 11,000,000 or higher.

One or more embodiments include the loop reactor of any precedingparagraph, wherein two loop reaction zones are formed, each loopreaction zone is formed by four vertical sections, four horizontalsections, and eight elbow sections and wherein the two loop reactionzones are connected to continuously transfer the slurry containedtherein.

In one or more embodiments, the loop reactor includes a plurality ofvertical sections; a plurality of elbow sections connecting the verticalsections to either a horizontal section having a horizontal length(L_(H)) or another elbow section, at least one elbow section having aninternal diameter (d), a radius (R_(c)) of an inner curvature and achord length (W) and wherein the horizontal length (L_(H)) is from 0feet to 3 feet and the chord length (W) is 250 inches or less; and atleast one loop reaction zone configured to polymerize an olefin monomerin the presence of a liquid diluent into a slurry including particles ofa polyolefin polymer, wherein the at least one elbow section isconfigured to maintain a Dean number (D_(n)) of the slurry flowingtherein to be higher than 3,000,000, where D_(n)=ρVd/μ*(d/2R_(c))^(1/2)and where ρ is a density of the slurry, V is a circulation velocity ofthe slurry, and p is a dynamic viscosity of the slurry.

One or more embodiments include processes for olefin polymerization in aloop reactor. In one or more embodiments, the process generally includespolymerizing an olefin monomer in the presence of a liquid diluent intoa slurry including particles of a polyolefin polymer inside at least oneloop reaction zone of a loop reactor, wherein the loop reactor includesthe loop reactor of any preceding paragraph.

One or more embodiments include the process of the preceding paragraph,wherein the polyolefin polymer is selected from polypropylene,polyethylene, and combinations thereof.

One or more embodiments include the process of any preceding paragraph,wherein the polyolefin polymer comprises polyethylene.

The above paragraphs present a simplified summary of the presentlydisclosed subject matter in order to provide a basic understanding ofsome aspects thereof. The summary is not an exhaustive overview, nor isit intended to identify key or critical elements to delineate the scopeof the subject matter claimed below. Its sole purpose is to present someconcepts in a simplified form as a prelude to the more detaileddescription set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. Advantages of the invention may become apparent to one ofskill in the art upon reading the following detailed description andupon reference to the following drawings.

FIG. 1 illustrates a block flow diagram depicting an exemplarypolyolefin production system having one or more loop reactors forproducing polyolefin in accordance with embodiments of the presenttechniques.

FIG. 2 illustrates an exemplary reactor system that can be used in thepolyolefin production system of FIG. 1 in accordance with embodiments ofthe present techniques.

FIG. 3A illustrates a loop reactor that can be used in a portion of thepolyolefin production system of FIG. 1 in accordance with embodiments ofthe present techniques.

FIG. 3B illustrates a loop reactor that can be used in a portion of thepolyolefin production system of FIG. 1 in accordance with alternateembodiments of the present techniques.

FIG. 3C illustrates a portion of a loop reactor within the polyolefinproduction system of FIG. 1 in accordance with alternate embodiments ofthe present techniques.

FIG. 3D illustrates an arc with its chord length (W) and height (H),where the arc is formed by the inner curvatures of two identical elbowsections.

FIG. 4 illustrates an exemplary process flow diagram of a method ofoperating a polyolefin manufacturing system in accordance withembodiments of the present techniques.

DETAILED DESCRIPTION

Illustrative embodiments of the subject matter claimed below will now bedisclosed. In the interest of clarity, not all features of an actualimplementation are described in this specification. It will beappreciated that in the development of any such actual embodiment,numerous implementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a developmenteffort, even if complex and time-consuming, would be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Particular embodiments of the invention may be described below withreference to block diagrams and/or operational illustrations of methods.It will be understood that each block of the block diagrams and/oroperational illustrations, and combinations of blocks in the blockdiagrams and/or operational illustrations, can be implemented by analogand/or digital hardware, and/or computer program instructions. Suchcomputer program instructions may be provided to a processor of ageneral-purpose computer, special purpose computer, ASIC, and/or otherprogrammable data processing system. The executed instructions maycreate structures and functions for implementing the actions specifiedin the block diagrams and/or operational illustrations. In somealternate implementations, the functions/actions/structures noted in theFigures may occur out of the order noted in the block diagrams and/oroperational illustrations. For example, two operations shown asoccurring in succession, in fact, may be executed substantiallyconcurrently or the operations may be executed in the reverse order,depending upon the functionality/acts/structure involved.

In the description below, unless otherwise specified, all compoundsdescribed herein may be substituted or unsubstituted and the listing ofcompounds includes derivatives thereof. Further, various ranges and/ornumerical limitations may be expressly stated below. It should berecognized that unless stated otherwise, it is intended that endpointsare to be interchangeable. Further, any ranges include iterative rangesof like magnitude falling within the expressly stated ranges orlimitations.

Embodiments described herein include the reduction of the footprint ofone or more loop reactors in a large-scale polyolefin production system,while maximizing the settling concentration of the recovered slurryproduction and maintaining the flow rate and the velocity of the slurrycirculating within the loop reactors, thereby significantly reducing thecapital cost for polyolefin production. In one embodiment, significantimprovement of the configuration of at least a loop reaction zone withina loop reactor is provided such that the vertical dimension (e.g., the Zaxis or the axis parallel to earth gravity) of the loop reactor remainsan efficient configuration for large scale, high capacity polyolefinproduction, while the horizontal dimension (e.g., the X-Y plane of aphysical land property that the loop reactors occupy) is significantlyreduced to save land space and capital cost. In another embodiment, atleast a horizontal section of the loop reactor is reduced and optimizedto reduce reactor footprint and capital cost, while maintaining slurrycirculation in the elbow section.

FIG. 1 depicts one example of a polyolefin production system 100 forproducing polymers, such as polyolefin and the like. The polyolefinproduction system 100 may generally include a wet reaction end 42 and adry finish end 44, which are connected to a control system 46 to processone or more feedstocks 14 (e.g., monomer reactants, diluents, catalystsand/or other chemical compounds) into reaction products (e.g., a slurry,a polyolefin fluff, a polyolefin pellet 38, and the like). Thefeedstocks 14 may be provided from various suppliers 12 (e.g., viapipelines, ships, trucks, cylinders, drums, and so forth), and suppliedto the polyolefin production system 100 via off-site and/or on-sitefacilities, (e.g., olefin plants, refineries, catalyst plants, and thelike).

The wet reaction end 42 of the polyolefin production system 100generally includes a feed system 16, a reactor system 20, a recoverysystem 24 and a fractionation system 30 and is adapted to process thefeedstocks 14 into a slurry and/or a polyolefin fluff stream 28 in wetor liquid phase. The dry finish end 44 is connected to or in fluidcommunication with the wet reaction end 42, and may include an extrusionsystem 36 and a loadout system 39 to process the polyolefin fluff 28 indry phase into a polyolefin pellet 38. The polyolefin pellet 38 is thendelivered into one or more customers 40.

The feed system 16 is adapted to supply the feedstocks 14 into thereactor system 20 via one or more feed lines 18. The feed system 16 isconfigured to meter and control the addition rate of the feedstocks 14into the feed lines 18 for maintaining the desired reactor stability andachieving the desired polyolefin properties or production rate. The feedsystem 16 may also be configured to receive, store, treat, and meter oneor more monomers or diluents, which are recovered as reactor effluentstreams from the recovery system 24 and recycled into the reactor system20. A plurality of the feed lines 18 may be used to feed various streamsof monomer, comonomer, diluent, catalysts, co-catalysts, hydrogen,additives, or combinations thereof to the reactor system 20. The feedlines 18 may tap into the wall of the polymerization loop reactor withinthe reactor system 20. In general, a single feed system may be dedicatedto a particular loop reactor. Alternatively, a plurality of the feedsystems 16 may be used in the reactor system and coupled to one or moreloop reactors disposed and operated in series or in parallel. Further,the feed system 16 may optionally receive recycled, non-polymercomponents from one or more downstream processing systems.

Exemplary feedstocks 14 may include, but are not limited to, olefinmonomers, comonomers, diluents, catalysts, co-catalysts, activators,chain transfer agents, molecular-weight control agents, co-reactants,additives, and combinations thereof. Suitable olefin monomers andcomonomers may include ethylene, propylene, butene, hexene, octene,decene, and combinations thereof, among others. The olefin monomer andcomonomers may be present in a liquid, gaseous, or supercritical fluidphase, depending on the type of the reactor system 20 used. Such olefinmonomers and comonomers are generally 1-olefins having up to 10 carbonatoms per molecule and may include no branching nearer the double bondthan the 4-position, such as 1-pentene, 1-hexene, 1-octene, and1-decene.

Suitable diluents may include propane, isobutane, n-butane, n-pentane,i-pentane, neopentane, cyclopentane, methylcyclopentane,ethylcyclopentane, n-hexane, cyclohexane, n-heptane, and combinationsthereof, among others.

The expression “chain transfer agent”, or alternatively“molecular-weight control agent”, as used herein, shall be understood tomean an agent that will provide an active hydrogen or halogen that canbe abstracted during a polymerization reaction. Chain transfer reactionsstop a growing radical chain during the polymerization and start a newone in its place. Thus, chain transfer results in shorter chains, lowerdegrees of polymerization, and lower molecular weights. One example of asuitable chain transfer agent or molecular-weight control agent ishydrogen; however, other chain transfer agents may be used in themethods and processes of the present invention, as the chain transferagents may vary with the type of polymerization involved. Otherexemplary chain transfer agents include but are not limited tomercaptans, aromatic compounds with benzylic hydrogens, alkyl halides,and halogenated hydrocarbons (such as carbon tetrachloride and carbontetrabromide).

Suitable catalysts may include Ziegler-Natta catalysts, chromiumcatalysts, metallocene catalysts, and combinations thereof, amongothers. The one or more catalysts fed into the reactor system 20 may beheterogeneous, homogenous, supported, and/or unsupported. Suitableco-catalysts may include borates, tri-ethylboron, methyl aluminoxane,triethylboron, methyl aluminoxane, borates, organoaluminum compounds(such as tri-ethylaluminum and tri-isobutylaluminum, triethylaluminum),and combinations thereof, among others. Suitable activators orco-catalysts may include solid super acids, additives, and combinationsthereof, among others.

In FIG. 1, the feed system 16 may include storage tanks, vessels,cylinders, and other containers to store the feedstocks 14 therein. Thefeed system 16 may also include treatment beds, such as molecular sievebeds, aluminum packing, etc., to process or treat the feedstocks 14prior to being fed into the feed lines 18. Any unwanted componentspresent within the feedstocks 14 may be removed after treatment.Examples of unwanted components within the feedstocks 14 includecatalyst poisons, such as water, oxygen, carbon monoxide, carbondioxide, and organic compounds containing sulfur, oxygen, and/orhalogens, among others. In other embodiment, the feedstocks 14 may befed directly into the reactor system 20 without being stored in the feedsystem 16. For example, ethylene monomer can be fed directly into thereactor system 20 without intermediate storage in the feed system 16.

The feed system 16 may be configured to prepare or condition thefeedstocks 14, such as catalysts, prior to being fed into the reactorsystem 20. For example, a catalyst may be prepared in catalystpreparation tanks to be mixed with a diluent (e.g., isobutane or hexane)or mineral oil. Furthermore, in operation, the feed system 16 may alsostore, treat, and meter recovered reactor effluent for recycle to thereactor. Indeed, operations in the feed system 16 generally receive bothfeedstock 14 and recovered reactor effluent streams.

The reactor system 20 may include one or more loop reactors connectedtogether and is adapted to carry out a polymerization reaction therein.For example, one polymerization reaction may be the addition andpolymerization of olefin monomers and comonomers into a long chainpolyolefin polymer compound in the presence of diluents, catalystsand/or other chemicals. In one embodiment, the loop reactors within thereactor system 20 are designed and uniquely configured to increasepolyolefin production capability, operation flexibility, and efficiency,and reduce system footprint and production cost.

In one example, the reactor system 20 within the polyolefin productionsystem 100 is configured to operate a plurality of loop polymerizationreactors connected and operated in series and/or in parallel, as acoupled operation of the loop reactor and/or as a decoupled orindependent operation. This capability to shift operation of a set ofpolymerization loop reactors between a series operation and a paralleloperation provides flexibility in producing mono-modal and/or multimodal(e.g., bimodal) polyolefin polymers, and in scheduling reactor shutdownand maintenance flexibility. In certain embodiments, at least two slurryloop reactors are run in series and then decoupled to run in parallel ordecoupled with one reactor run while the other reactor is down formaintenance. This gives the plant flexibility to produce bimodalpolyolefin polymer products and switch to single polyolefin polymerproduct for varying markets conditions. The parallel reactors could runon separate parallel feed lines and peripheral recovery systems orshared feed systems and recovery systems.

In one embodiment, the reactor system 20 may include one or more slurrypolymerization reactors. The polymerization reactors may be of the sametype or different types, and arranged serially or in parallel, toproduce a polyolefin particulate product, such as the polyolefin fluff28, generically referred to as “fluff” herein. To facilitateexplanation, the following examples are limited in scope to specificreactor types believed to be familiar to those skilled in the art and tocombinations. To one of ordinary skill in the art using this disclosure,however, the present techniques are applicable to more complex reactorarrangements, such as those involving additional reactors, differentreactor types, and/or alternative ordering of the reactors or reactortypes, as well as various diluent and monomer recovery systems andequipment disposed between or among the reactors, and so on. Sucharrangements are considered to be well within the scope of the presentinvention.

One reactor type include reactors within which polymerization occurswithin a liquid phase. Examples of such liquid phase reactors includeautoclaves, boiling liquid-pool reactors, loop slurry reactors (verticalor horizontal), and so forth. For simplicity, a loop slurry reactor forproducing polyolefin, such as polyethylene or polypropylene, isdiscussed in the present context though it is to be understood that thepresent techniques may be similarly applicable to other types of liquidphase reactors.

In another embodiment, the reactor system 20 may include one or moreloop reactors connected together into at least one loop reaction zonefor carrying a polymerization reaction. Each loop reactor may includefour vertical sections, four horizontal sections, and eight elbowsections. In another example, each loop reactor may include six verticalsections, six horizontal sections, and twelve elbow sections. In stillanother example, one loop reactor may include two loop reaction zones,where at least one loop reaction zone is formed by four verticalsections, four horizontal sections, and eight elbow sections, whileanother loop reaction zone is formed by other suitable combination ofvertical sections and horizontal sections that are connected by elbowsections. Exemplary reactor system configurations, loop reactors andloop reaction zones for slurry polymerization includes those disclosedin U.S. Pat. No. 6,239,235, U.S. Pat. No. 7,033,545, and U.S. PatentApplication Publication No. 2011/0288247, each of which is incorporatedherein by reference in its entirety for its description of loop reactorsand their diameters, lengths, equipment, and operation.

FIG. 2 depicts a reactor system 200 as one example of the one or moreloop reactors within the reactor system 20. The reactor system 200 mayinclude two or more loop reactors 50A, 50B connected together into oneor more loop reaction zones and can be flexibly switched betweenoperations in series or in parallel for producing polyolefin. The loopreactors 50A, SOB may include a plurality of vertical sections 210 andhorizontal sections 220 (e.g., upper horizontal sections and lowerhorizontal sections, etc., where these upper and lower horizontalsegments define upper and lower zones of horizontal flow). For example,the loop reactor 50A, 50B may include eight to sixteen or other numberof the vertical sections 210, such as jacketed vertical pipe legs,approximately 24 inches in diameter and approximately 200 feet inlength.

As shown in FIG. 2, the vertical section 210 and the horizontal section220 are connected by a smooth bend (e.g., an elbow section 230), thusproviding a continuous flow path substantially free from internalobstructions among the one or more loop reaction zones within the loopreactors 50A, 50B of the reactor system 200. In one example, an elbowsection can be used to connect a terminal end of a horizontal section toa terminal end of the closest vertical section (for convenience andclarity, only some of the vertical sections, horizontal sections andelbow sections are numbered in FIG. 2). For example, two elbow sections230 may be used to connect the top and bottom portions of each verticalsection 210 with another horizontal section 220 or another elbow section230. As another example, two elbow sections 230 may be used to connectboth ends of a horizontal section 220 with two vertical sections 210. Ingeneral, two identical elbow sections are formed into an arc (of ahypothetical circle, where an inner curvature 238 of each elbow section230 forms half of the arc of the hypothetical circle, and thehypothetical circle has a radius (R_(c)) as shown in FIG. 3C,). Eachvertical section 210 has a vertical length (L_(V)), and each horizontalsection 220 has a horizontal length (L_(H)). Note that the respectivevertical lengths (L_(V)) and horizontal lengths (L_(H)) do not have tobe the same.

In general, the polymerization reaction within the reactor system 200 isexothermic and removal of the heat of reaction is required. The loopreactors 50A, SOB may be cooled by means of a heat exchanger. Forexample, each leg or vertical section 210 may be protected andsurrounded by a heat exchanger, such as a cooling jacket 52 or othersuitable pipe-shaped heat exchangers, configured to surround thevertical sections 210 and/or other sections. FIG. 2 illustrates twofour-legged reactors, each with 4 vertical sections arranged vertically.The vertical section 210 surrounded with the cooling jacket 52 couldalso be arranged horizontally. The cooling jackets 52 are configuredwith cooling medium (e.g., water, and other coolants) flowing therein toremove heat.

One or more feed lines 58A, 58B are used to introduce monomers (and/orco-monomers, if any), one or more diluents, and/or other chemicalcomponents into the loop reactors 50A, 50B, respectively. The feed lines58A, 58B may be connected to the loop reactors 50A, 50B directly at oneor more locations or can be combined and fed into a single line, such asa condensed diluent recycle line. In addition, one or more feed lines 60are used to introduce catalysts and other additives into the loopreactors 50A, SOB. The feed lines 58A, 58B, 60 generally correspond tothe feed line 18 of FIG. 1.

Reaction conditions, such as temperature, pressure, and reactantconcentrations, in each loop reactor 50A, 50B are regulated tofacilitate desired properties and production rate of the polyolefinpolymer products generated therein, and control stability of the loopreactors 50A, SOB. Temperature is typically maintained below a level atwhich the polymer product would go into solution, swell, soften, orbecome sticky. Due to the exothermic nature of the polymerizationreaction, a cooling fluid may be circulated through the cooling jackets52 around portions of the loop slurry reactor 50A, 50B to remove excessheat, thereby maintaining the temperature within a desired range,generally between about 150° F. and about 250° F. (about 65° C. to about121° C.). Likewise, pressure in each loop reactor 50A, 50B may beregulated within a desired pressure range, generally in a range betweenabout 100 psig and about 800 psig, such as from about 450 psig to about700 psig. In one example, a monomer of ethylene and a comonomer of1-hexene can be polymerized at a reaction temperature into apolyethylene polyolefin polymer, which is substantially insoluble in afluid medium, thereby forming a slurry of solid particulates therein.

Further, the loop reactors 50A, 50B may also include other mechanisms orinstruments to measure and/or control process variables, such astemperature, pressure, flow rate of the feedstock, slurry density, speedof the fluid, speed of the circulating slurry, and so forth. Suchinstrumentation may include one or more sensors or sensing elements,transmitters, and so forth, located within or outside of the reactors50A, 50B, as appropriate. For a pressure control mechanism, a sensingelement, for example, a diaphragm, may be used. For a temperaturecontrol instrument, a sensing element, such as a thermocouple, aresistance temperature detector (RTD), and the like, of which may behoused in a thermowell, for instance, may be used.

Various instruments may have local indication of the sensed processvariables. For instance, a pressure control instrument may have a localpressure gauge and a temperature control instrument may be or have alocal temperature gauge, both of which may be read locally by anoperator or engineer, and controlled by the control system 46, forexample. Transmitters can be used to convert a received analog signalfrom a sensing element to a digital signal for feed or transmission to acontrol system, for example, the control system 46.

The loop reactors 50A, 50B are used to carry out polyolefin (e.g.,polyethylene (PE), polypropylene (PP)) polymerization under slurryconditions in which insoluble particles of polyolefin polymers areformed in a fluid medium and suspended as in slurry phase. The olefinmonomers, the co-monomers, and the like may be polymerized in thepresence of catalysts, liquid diluents, and the like into a mixture of afluid slurry. The fluid slurry may include solid particles of one ormore polyolefin polymers, the monomers, the liquid diluents, thediluents and the like.

The mixture of the polymerizing fluid slurry is circulated by means ofan impeller (not shown), which is generally disposed within the interiorof the loop reactors 50A, 50B to create a turbulent mixing zone withinthe fluid slurry and maintain a relatively constant slurry velocityand/or mass flow therein. The impellers within the loop reactors 50A,50B may be driven by one or more motors 56A, 56B, which are coupled to ahigh performance pump, such as pump 54A, 54B. The pump 54A, 54B areconnected to the impellers within the loop reactors 50A, 50B,respectively. Examples of pumps include in-line axial flow pumps andmixed-flow pumps. The impellers may also assist in propelling the fluidslurry through the loop reaction zone within each loop reactor atsufficient speed to keep solid particulates, such as catalysts orpolyolefin polymer product particles, suspended within the fluid medium.

As shown in FIG. 2, the fluid slurry can be discharged from the loopreactors 50A, SOB, via a product slurry line 27, a slurry transfer line21, or a fluff slurry product line 22, which are connected to orconfigured as one or more settling legs, valve mechanisms (e.g., Ramvalves, modulating valves), and/or continuous take-off (CTO) assemblies(coupled with one or more Ram valves, modulating valves, or other valveconfigurations), among others.

The slurry transfer line 21 is configured to discharge the fluid slurryfrom the loop reactor 50A directly (e.g., via one or more product slurrylines 27) or into the loop reactor 50B (e.g., via a transfer line 21L)through a settling leg, an isolation valve (e.g., a Ram valve), acontinuous take-off (which includes an isolation (Ram) valve and amodulating valve), or other valve configuration. The fluid flow withinthe slurry transfer line 21 can be regulated by one or more valvemechanisms and/or CTO assemblies coupled thereto. Alternatively, theslurry transfer line 21 may not be modulated and can function as acontinuous slurry transfer line. In certain embodiments, the slurrytransfer line 21 is rerouted to one of the product slurry line 27. Inalternate embodiments, one product slurry line 27 is connected to theloop reactor 50A, for example, near one of the elbow sections 230, andthe slurry transfer line 21 can be closed.

In one example, the fluid slurry discharged from the slurry transferline 21 of the loop reactor 50A is continuous and not directlymodulated. No CTO or settling leg is employed. Instead, the slurrytransfer line 21 is coupled to a full-bore Ram valve maintained in afull-open position, and not additionally through a modulating valve. Inanother example, the slurry transfer line 21 is coupled to an isolationvalve (e.g., a Ram valve or the like) positioned at the reactor wall andwithout a modulating valve. The Ram valve is provided to isolate theslurry transfer line 21 from the loop reactor 50A, if so desired. Instill another example, a CTO with a modulating valve may be situated atthe slurry transfer line 21 of the loop reactor 50A. If so included, themodulating valve may control flow rate of the fluid slurry transferredtherein and facilitate pressure control within the loop reactor 50A.

In another example (not illustrated), a modulating valve may be disposeddownstream on the slurry transfer line 21. In still another example, aRam valve may be positioned at the outlet of the slurry transfer line 21connected to the wall of the loop reactor 50B to provide for isolationof the slurry transfer line 21 from the loop reactor 50B, when suchisolation is desired. It may be desired to isolate the slurry transferline 21 from the loop reactors 50A, 50B during maintenance or downtimeof the reactor system 200, or when an alternate discharge or transferline from the loop reactor 50A is placed in service, and so on. Theoperation of the Ram valves may be manually controlled,hydraulic-assisted, air-assisted, remote-controlled, automated, and soon. The transfer line 21L can be manually removed from service (e.g.,manually closing the Ram valves) or automatically removed (e.g., via acontrol system automatically closing the Ram valves) from service.

The fluff slurry product line 22 is configured to discharge the fluidslurry from the loop reactor 50B via a flow control valve 25 (e.g., amodulating valve) and into the recovery system 24 (which is also shownin FIG. 1). The fluff slurry product line 22 may be connected, directlyor indirectly, to a settling leg, a continuous take-off (CTO) assembly,or other valve configurations to discharge the fluid slurry. The fluidslurry may be discharged intermittently, such as through a settling legconfiguration. Alternatively, the fluid slurry may be dischargedcontinuously, such as through a CTO assembly. A variety of dischargeconfigurations are contemplated for a continuous discharge. For example,an isolation valve (e.g., full-bore Ram valve) without an accompanyingmodulating valve may be used for continuous discharge of the fluidslurry from the loop reactors 50A or 50B.

Pressure elements or instruments may be disposed on the loop reactors50A, 50B and on the slurry transfer line 21. In some examples, thepressure in the loop reactor 50A may float on the pressure in the loopreactor 50B. The loop reactors 50A, 50B may be maintained at the same,similar, or different pressure. The inlet position of the slurrytransfer line 21 may be coupled to the loop reactor 50A on the dischargeside of the pump 54A in the loop reactor 50A. The outlet position of theslurry transfer line 21 may be coupled to the loop reactor 50B on thesuction side of the pump 54B in the loop reactor 50B. Such aconfiguration may provide a positive pressure differential (i.e., adriving force) for the fluid slurry flowing through the slurry transferline 21 from the loop reactor 50A to the loop reactor 50B. In oneexample, a pressure differential (provided from the discharge of thepump 54A to the suction of the pump 54B) is about 20 pounds per squareinch (psi).

One embodiment of the invention provides that a polymerization reactionis carried out in a single loop reaction zone within the polyolefinproduction system 100 and the fluid slurry is continuously deliveredthrough the loop reactors 50A, 50B and discharged into the fluff slurryproduct line 22. In another embodiment, the polymerization reactionwithin the polyolefin production system 100 is carried out in two loopreaction zones within the loop reactors 50A, 50B, respectively, and thefluid slurry is transferred from one loop reaction zone within the loopreactor 50A into another loop reaction zone within the loop reactor 50B,each reaction zone with its individual process variables.

A further embodiment of the invention provides significant improvementof the configuration of the vertical sections 210, horizontal sections220, and the elbow sections 230 within the loop reaction zone of theloop reactors 50A, 50B of the polyolefin production system 100. Thesystem footprint and support structure of at least one loop reactor isreduced, thereby increasing production efficiency and saving capitalcost, while maintaining a high processing capacity. In one embodiment ahorizontal length (L_(H)) of at least one horizontal section is greatlyreduced. In another embodiment, at least one elbow section of thereactor system is configured to maintain a Dean number (D_(n)) of theslurry flowing therein to be higher than 3,000,000. At the same time,the vertical sections of the loop reactors 50A, 50B in the polyolefinproduction system 100 are configured to maintain its capacity for atotal processing volume of the loop reactor measured at more than about10,000 gallons, such as about 20,000 gallons or more, about 40,000gallons or more, or about 50,000 or more. An exemplary nominal capacityfor the exemplary production system 10 is about 700-1600 million poundsof polyolefin produced per year. Exemplary hourly design rates areapproximately 70,000 to 200,000 pounds of polymerized/extrudedpolyolefin per hour. It should be emphasized, however, that the presenttechniques apply to polyolefin manufacturing processes includingpolyethylene production systems having nominal capacities and designrates outside of these exemplary ranges

FIGS. 3A and 3B depict examples of loop reactors 300, 310 that can beused in such polyolefin production system for olefin polymerization. Inone aspect, it is contemplated to configure at least one of the elbowsections 230 and maintain a Dean number (D_(n)) of the slurry flowingwithin the elbow section 230 to be higher than 3,000,000, where theelbow section 230 contains an internal diameter (d) and a radius (R_(c))of an inner curvature. In one embodiment, the Dean number is measured asρVd/μ*(d/2R_(c))^(1/2), where ρ is a density of the fluid slurry asmeasured in an unit of, for example, lb/ft3, where V is the circulatingvelocity of the slurry in the elbow section as measured in an unit of,for example, meters per second, where d is the internal diameter of theelbow section 230 as measured in an unit of, for example, feet orinches, where p is a dynamic viscosity of the fluid slurry as measuredin an unit of, for example, lb/ft/sec, where R_(c) is a radius of aninner curvature of an elbow section 230 as measured in an unit of, forexample, meters or feet. In another aspect, the vertical dimension(e.g., the Z axis or the axis parallel to earth gravity) of the loopreactors remains an efficient configuration for large scale, highcapacity polyolefin production, while the horizontal dimension (e.g.,the X-Y plane of a physical land property that the loop reactors occupy)is significantly reduced to save land space and capital cost.

As shown in FIGS. 3A-3D, one embodiment of the invention provides thatthe loop reactors 300, 310 may be configured to include a verticallength (L_(V)) of at least one of the vertical sections 210 to be muchlonger than a horizontal length (L_(H)) of at least one of thehorizontal sections 220. For example, the vertical length (L_(V)) of atleast one of the vertical sections 210 in a loop reaction zone of theloop reactors 50A, 50B, 300, 310, may be 3 times (3×) or larger than thehorizontal length (L_(H)) of at least one of the horizontal sections220, such as 60 times (60×) or larger, 80 times (80×) or larger, 100times (100×) or larger, 150 times (150×) or larger, or 250 times (250×)or larger. In one example, the vertical length (L_(V)), as measured fromthe lowest Z axis value of a vertical section 210 to the highest Z axisvalue of the vertical sections 210 can be about 60 feet or larger, suchas about 100 feet or larger, about 150 feet or larger, or about 300 feetor larger. In some embodiments, the vertical length (L_(V)) is betweenabout 190 feet and 225 feet, between about 225 feet and 260 feet, orbetween about 260 feet and 300 feet. The vertical section 210 may beconnected to the horizontal section 220 via at least one elbow section230.

In another aspect, it is contemplated that the horizontal length (L_(H))of at least one of the horizontal sections 220, next to the verticalsection 210 and connected by a neighboring elbow section, issignificantly reduced. As an example, the horizontal length (L_(H)) ofthe horizontal section 220 of the loop reactors 50A, 50B, 300, 310 asprovided herein and as measured from one terminal end to anotherterminal end of the horizontal section 220 in the X-Y plane of aphysical land property may be about 6 feet or smaller, such as about 3feet or smaller, such as between about 1 feet and about 3 feet, e.g.,about 18 inches, about 24 inches. As another example, the horizontallength (L_(H)) of at least one horizontal section can be configured tobe zero, where the loop reactor contains no horizontal section at alland the vertical sections 210 are connected to the elbow sections 230,where two elbow sections are formed into an arc having a chord length(W) and a height (H), which forms into a perpendicular angle α. As usedherein, the chord length (W) is defined to be the distance between twoterminal ends of an arc, where the arc is formed by two identical elbowsections, i.e., two inner curvatures 238 a, 238 b of two elbow sections(as illustrated in detail in FIG. 3D). In such case, one elbow sectionis provided to connect a vertical section to another elbow section.

In FIG. 3A, the loop reactor 300 includes one or more continuoustake-off (CTO) assemblies 234. Each CTO assembly 234 can be connected toan associated fluff slurry product line 22, as shown in FIGS. 1-2. Otherconfigurations, for example, a configuration where multiple CTOassemblies feed a single fluff slurry product line 22 are contemplatedwithin the scope of the present disclosure. The CTO assemblies 234 maybe configured to extend from at least one of the elbow sections 230, asshown in FIG. 3A. Alternatively, the CTO assemblies 234 may beconfigured to extend from at least one of the horizontal section 220.

The CTO assemblies 234 may include one or more valve mechanisms, such asan isolation valve (e.g., a Ram valve), a modulating valve (e.g., av-ball valve), or other valve configuration. In one example, the CTOassemblies 234 may include a modulating valve to control the continuousdischarge of the fluid slurry from the loop reactor. In another example,the CTO assemblies 234 may include an isolation valve (e.g., a Ramvalve) and an accompanying modulating valve (e.g., a v-ball valve) forcontinuous discharge of the fluid slurry. A Ram valve in a closedposition may beneficially provide a surface that is flush with the innerwall of the reactor to preclude the presence of a cavity, space, or voidfor polymer to collect when the Ram valve is in the closed position.

The CTO assemblies 234 may be located in or adjacent to a downstream endof one of the lower horizontal section or the neighboring elbow section.In one configuration, the continuous take-off (CTO) assemblies 234 arecoupled to the fluff slurry product line 22 and connected to extend fromat least one of the elbow sections 230 to help recover solid polyolefinproduct particles, without affecting the circulating velocity of thefluid slurry. In another configuration, the CTO assembly 234 may be anelongated hollow appendage coupled to the transfer slurry line 21 andconfigured to continuously take off an intermediate product slurry.

In addition, one or more CTO assemblies 234 can be positioned in an areanear the last point within the loop reaction zone, where the flow of theslurry turns upward. In another configuration, the CTO assemblies 234can be positioned prior to a catalyst feed line, such as the feed line60, so as to allow a maximum possible retention time for a catalystfreshly introduced into the loop reactor 300 before the catalyst passesa take-off point (i.e., the location of the CTO assembly 234) for thefirst time.

In general, the continuous take off assemblies 234 can be positioned onany horizontal, vertical, or elbow sections within the loop reactor 300.Also, the section of the loop reactor 300, to which the CTO appendage isattached, can be of a larger diameter to slow down the flow of the fluidslurry flowing therein and hence further allow stratification of thefluid flow so that the product coming off can have an even greaterconcentration of solids.

In operation, depending on the positioning of the discharge valve on theloop reactor, for example, a discharged fluid slurry having a greatersolids concentration than the fluid slurry circulating within the loopreactor may be realized with a discharge configuration having anisolation valve (Ram valve) alone, or having a CTO configuration with anisolation valve (Ram valve) and a modulating valve. Exemplary CTOconfigurations and other discharge configurations may be found in U.S.Patent Application Publication No. 2011/0288247, and in U.S. Pat. No.6,239,235. Both are incorporated herein by reference in their entirety.In certain examples, the CTO assemblies 234 may have a Ram valveconnected to the wall of the loop reactor, and a modulating flow controlvalve (e.g., v-ball control valve) connected to the discharge fluid line(e.g., the fluff slurry product line 22, the transfer slurry line, etc.)In an alternate embodiment, the product fluff slurry may be dischargedthrough a settling leg configuration in lieu of a CTO.

In FIG. 3B, the loop reactor 310 includes one or more settling legassemblies 232 configured to extend from at least one of the horizontalsections 220 and coupled to the fluff slurry product line 22. Eachhorizontal section 220 may include a single settling leg or multiplesettling legs as long as the horizontal space permits. The settling legsare generally connected to the reduced horizontal dimension of thehorizontal section 220.

Alternatively, the settling leg assemblies 232 may be configured toextend from the elbow sections 230. The settling leg assemblies 232 mayinclude one or more valve mechanisms, such as an isolation valve (e.g.,a Ram valve) or other valve configurations. The valves within thesettling leg assemblies 232 are configured to continuously settle andperiodically withdraw the fluid slurry. Also, the valves within thesettling leg assemblies 232 can be used to accumulate the fluid slurrysettled therein and periodically withdraw the fluid slurry into thetransfer slurry line 21 and/or the fluff slurry product line 22.

As shown in FIGS. 3A-3B, the loop reactors 300, 310 may include at leastone loop reaction zone formed of four vertical sections 210, fourhorizontal sections 220, and eight elbow sections 230 connectedtogether. Alternatively, the loop reaction zone is formed of sixvertical sections, six horizontal sections, and twelve elbow sectionsconnected together. Loop reactions zones with more than six vertical andhorizontal sections and more than twelve elbow sections are also withinthe scope of this invention. Most importantly, the configuration of theloop reactors 300, 310 are designed to take advantage of verticalgravity pull, one or more high performance circulation pumps coupledthereto (e.g., the motors 56A, 56B, the pump 54A, 54B, or other suitablepumps, and the CTO assembly 134 (or continuous settling legs) to helpincrease the flow rate and the velocity of the fluid slurry circulatingwithin the loop reactors 300, 310, leading to high concentration of thecirculating slurry, and ultimately high yield of the solid particleproducts taking off from the circulating slurry.

FIG. 3C depicts an exemplary portion of a loop reactor and the adjacentelbow section 230 within the polyolefin production system 100. The loopreactor can be any of the loop reactors 50A, 50B, 300, 310. The loopreactor may include a plurality of elbow sections 230, each elbowsection 230 having an internal diameter (d) and a radius (Re) of aninner curvature 238. The flow of the fluid slurry within the elbowsection 230 is indicated by an arrow “F”. Assuming two inner curvaturesof two elbow sections 230 form an arc, as illustrated in FIG. 3D as aninner curvature 238 a and an inner curvature 238 b, each elbow section230 has a height (H) measured from a midpoint of the arc. In addition,each of the inner curvature 238 a and the inner curvature 238 b has alength, which is half of the chord length (W) of the arc, i.e., W/2.Accordingly, an inner curvature 238 of the elbow section 230 as shown inFIG. 3C has a length of W/2.

As shown in the example in FIG. 3C, the elbow section 230 (e.g., atleast one elbow section within any of the loop reactors 50A, 50B, 300,310) may be connected to an elbow flow meter 370 (sometimes referred toas a Smart Ell). For example, an inner pressure tap 362 and an outerpressure tap 364 may be positioned about the inner and outer walls ofthe elbow section 230 to detect and measure a pressure differentialbetween the inner and outer walls of the elbow section 230.

In one example, the inner pressure tap 362 and the outer pressure tap364 are flushed continuously with a diluent (e.g., isobutane or in somecase recycle isobutene) at a relatively high rate to prevent polymerslurry from plugging the components of the elbow flow meter 370. Inanother example, the inner pressure tap 362 and the outer pressure tap364 may include an inner diaphragm 352 and an outer diaphragm 354,respectively, at the inner and outer walls of the elbow section 230,such that the inner pressure tap 362 and the outer pressure tap 364 canbe protected from being plugged or fouled with polymer slurry. With theuse of the inner diaphragm 352 and the outer diaphragm 354, a diluentflush may not be necessary. The elimination of the diluent flush mayreduce the demand for an olefin-free diluent. Moreover, the eliminationof the diluent flush at the inner pressure tap 362 and the outerpressure tap 364 may generally improve the consistency of the pressuremeasurements obtained from the elbow flow meter 370.

In one embodiment, at least an elbow section within the loop reactionzone of the loop reactor is configured to minimize friction loss.Referring to FIG. 3C, for example, the radius (R_(c)) and the internaldiameter (d) of the elbow section 230 can be configured to minimizefriction loss of the fluid slurry within the elbow section 230. In oneexample, a ratio (R_(c)/d) of the radius (R_(c)) and the internaldiameter (d) of the elbow section 230 is maintained between about 1 andabout 10, or between about 2 and about 4. One of skill in the art wouldunderstand, particularly in light of the present disclosure, that themeasured parameters used to calculate the values of the radius (Re) andthe internal diameter (d) of the elbow section 230 and the R_(c)/d ratioreferenced herein must be converted to like or consistent units beforemaking the calculation, even that the units cancel each other out toresult in the R_(c)/d ratio.

In another embodiment of the present invention, the loop reactormaintains a high circulation velocity (V) and a high flow rate (e.g., ahigh Reynolds number, a high Dean number (D_(n)), etc.) for thecirculating fluid slurry, especially within the elbow section 230. Thecirculation velocity (V) may be measured, for example, by the elbow flowmeter 370 which may be coupled to the inner pressure tap 362 and theouter pressure tap 364 on the inner and outer walls, respectively, ofthe elbow section 230. In one example, the circulating velocity (V) ofthe slurry in the elbow section 230 is about 6 meters per second (6 m/s)or higher, such as about 9 m/s or higher. In another example, the Deannumber is maintained to be higher than 3,000,000, whereD_(n)=ρVd/μ*(d/2R_(c))^(1/2) and where ρ is a density of the fluidslurry as measured in an unit of, for example, lb/ft3, where V is thecirculating velocity of the slurry in the elbow section as measured inan unit of, for example, meters per second, where d is the internaldiameter of the elbow section 230 as measured in an unit of, forexample, feet or inches, and where μ is a dynamic viscosity of the fluidslurry as measured in an unit of, for example, lb/ft/sec. In anotherexample, a Reynolds number (N_(RE)) of the fluid slurry within the atleast one elbow section, where N_(RE)=ρVd/μ, can be maintained at about11,000,000 or higher.

Both Dean numbers and Reynolds numbers are dimensionless numbers. One ofskill in the art would understand, particularly in light of the presentdisclosure, that the measured parameters used to calculate thedimensionless numbers referenced herein must be converted to like orconsistent units before making the calculation such that the unitscancel each other out to result in the dimensionless number.

In FIG. 3C, a portion of the are of the inner curvature of the elbowsection 230 is shown in dotted curve line. In an embodiment, it iscontemplated to reduce the chord length (W) of the are of the innercurvature 238 in the elbow section 230. In one embodiment, the chordlength (W) of the inner curvature 238 in the elbow section 230 isconfigured to be about 250 inches or less. In another embodiment, theradius (R_(c)) of the inner curvature in the elbow section 230 isconfigured to be about 72 inches or less, as measured asR_(c)=H/2+W²/8H.

In addition, it is contemplated to adjust the dimension, variousparameters, and configuration of the elbow section 230 and reducefriction loss of the slurry flowing therein. In one embodiment, a ratio(R_(c)/d) of the radius (R_(c)) of the inner curvature and the internaldiameter (d) of the elbow section 230 is configured to be maintainedbetween about 1 and about 10, or between about 2 and about 4 to reducefriction loss.

Further, the circulation velocity (V) of the slurry flowing through theloop reaction zone within the reactors 300, 310 generally encounter nointerference within the vertical sections; accordingly, it iscontemplated to configure the dimension of the elbow section 230 inorder to maintain the circulation velocity (V) of the slurry flowingthrough the elbow section 230, where the circulation velocity (V) of theslurry correlates with the internal diameter (d) and the radius (R_(c))of the inner curvature of the elbow section 230. For example, thecirculation velocity (V) of the slurry flowing through the elbow section230 can be adjusted according to V equals to μ*D_(n) divided byρ*d*(d/2R_(c))^(1/2),

$V = \frac{\mu\; D_{n}}{\rho\;{d\left( \frac{d}{2R_{c}} \right)}^{1/2}}$

where μ is the dynamic viscosity of the slurry, D_(n) is the Dean numberof the slurry, ρ is the density of the slurry, d is the internaldiameter of the elbow section 230, and R_(c) is the radius of the innercurvature 238 of the elbow section 230. In one example, the circulationvelocity (V) of the slurry within the at least one loop reaction zone ismaintained at about 9 meters per second or higher. In another example,the circulation velocity (V) of the slurry within the at least one loopreaction zone is maintained at about 9 meters per second or lower. Stillfurther, a Reynolds number (N_(RE)) of the slurry flowing in the elbowsection 230 can also be maintained by adjusting the dimension, variousparameters and configuration of the elbow section. In one example, theReynolds number of the slurry within the elbow section 230 is maintainedat about 11,000,000 or higher.

FIG. 4 illustrates a process 400 of operating a polyolefin manufacturingsystem, such as the polyolefin production system 100 as shown in FIG. 1.The process 400 is provided for olefin polymerization in a loop reactionzone of a reactor system, such as the reactor system 20, 200 with one ormore loop reactors 50A, 50B, 300, 310. In the polyolefin productionsystem 100, one or more olefin monomers and optionally comonomers arepolymerized to form product polymer particulates, typically called fluffor granules. The fluff may possess one or more melt, physical,rheological, and/or mechanical properties of interest, such as density,melt index (MI), molecular weight, copolymer or comonomer content,modulus, and the like. The reaction conditions, such as temperature,pressure, flow rate, mechanical agitation, product takeoff, componentconcentrations, catalyst type, polymer production rate, and so forth,are selected to achieve desired fluff properties.

At step 410, an olefin monomer and a diluent are fed into a loop reactorsystem having a loop reaction zone therein. For example, monomers anddiluents supplied from the feedstocks 14 are fed through the feed system16 into the reactor system 20 via the feed lines 18. In one example, themonomer is ethylene and the comonomer is 1-hexene. In another example,the monomer is propylene and the comonomer is ethylene.

In the case of an ethylene monomer, an ethylene feedstock may besupplied via a feed line at approximately 800-1450 pounds per squareinch gauge (psig) at a temperature between about 45° F. and about 65° F.(about 7° C. and about 18° C.). A hydrogen feedstock may be supplied viaa separate feed line, but at approximately 900-1000 psig at atemperature between about 90° F. and about 110° F. (about 32° C. andabout 43° C.). Of course, a variety of supply conditions can be used fordelivering ethylene, hydrogen, and other feedstocks 14.

In addition to the olefin monomers and comonomers, a catalyst thatfacilitates polymerization of the ethylene monomer is added to the loopreactor. The catalyst may be a particle suspended in a fluid mediumwithin the loop reactor. In general, Ziegler catalysts, Ziegler-Nattacatalysts, metallocene catalysts, chromium catalysts, and otherwell-known polyolefin catalysts, as well as co-catalysts, may be used.Typically, an olefin-free diluent or mineral oil, for example, is usedin the preparation and/or delivery of the catalyst in a feed line (e.g.,the feed line 18) that taps into the wall of the polymerization reactor.Further, a diluent may be fed into the loop reactor, typically aliquid-phase loop reactor.

At step 420, the olefin monomer, which is supplied into a loop reactor,such as the loop reactors 50A, 50B, 300 310, is then polymerized in thepresence of a liquid diluent into a polyolefin-containing fluff slurry.The fluff slurry may contain solid particles of a polyolefin polymer.The diluent may be an inert hydrocarbon, such as isobutane, propane,n-butane, n-pentane, i-pentane, neopentane, n-hexane, cyclohexane,cyclopentane, methylcyclopentane, ethylcyclohexane, combinations thereofand the like, which is in its liquid phase at reaction conditions. Theuse of the diluent is to suspend the catalyst particles and polymerfluff into a slurry mixture within the loop reactor. The diluent, as itsname indicated, may also be used to flush the internal volume of theloop reactor or fluid lines, to mitigate plugging or fouling, tofacilitate flow of the polymer slurry in conduits and lines within theproduction system, and so on. Moreover, in examples of polypropyleneproduction, the propylene monomer itself may act as a diluent.

Each loop reactor may include a plurality of vertical sections and aplurality of elbow sections connecting the vertical sections to either ahorizontal section or another elbow section. In addition, at least oneelbow section may have an internal diameter (d) and a radius (R_(c)) ofan inner curvature, which can be adjusted maintain a Dean number (D_(n))of the slurry flowing therein to be higher than 3,000,000, whereD_(n)=ρVd/μ*(d/2R_(c))^(1/2) and where ρ is a density of the slurry, Vis a circulation velocity of the slurry, and μ is a dynamic viscosity ofthe slurry. Exemplary vertical section, horizontal section and elbowsection dimensions are shown in Tables 1-2.

TABLE 1 Chord length (W), height (H), and radius of curvature (R_(c)) ofexemplary elbow sections. W (Inches) H (Inches) R_(c) (inches) R_(c)(feet) 72 36 36 3.0 72 24 39 3.3 72 12 60 5.0 72 6 111 9.3 60 36 30.52.5 60 24 30.8 2.6 60 12 43.5 3.6 60 6 78.0 6.5 48 36 26.0 2.2 48 2424.0 2.0 48 12 30.0 2.5 48 6 51.0 4.3 36 36 22.5 1.9 36 24 18.8 1.6 3612 19.5 1.6 36 6 30.0 2.5 24 36 20.0 1.7 24 24 15.0 1.3 24 12 12.0 1.024 6 15.0 1.3 12 36 18.5 1.5 12 24 12.8 1.1 12 12 7.5 0.6 12 6 6.0 0.5

TABLE 2 Diameter (d), radius of curvature (R_(c)), R_(c)/d ratio, fluidflow rate (V), Reynolds number ((N_(RE)), and Dean number (D_(n)) valuesof exemplary elbow sections. d d d R_(c) R_(c) V V (meter) (inch) (feet)(inch) (feet) R_(c)/d (m/s) (feet/s) N_(RE) D_(n) 0.61 24.0 2.0 72 6 312.2 40 15,053,763 6,145,673 0.61 24.0 2.0 72 6 3 11.0 36 13,548,3875,531,106 0.61 24.0 2.0 72 6 3 10.4 34 12,795,699 5,223,822 0.61 24.02.0 72 6 3 9.8 32 12,043,011 4,916,539 0.61 24.0 2.0 72 10 5.0 9.1 3011,290,323 3,570,313 0.61 24.0 2.0 72 8 4.0 9.1 30 11,290,323 3,991,7320.61 24.0 2.0 72 6 3.0 9.1 30 11,290.323 4,609,255 0.61 24.0 2.0 60 52.5 9.1 30 11,290,323 5,049,186 0.61 24.0 2.0 48 4 2.0 9.1 30 11,290,3235,645,161 0.61 24.0 2.0 36 3 1.5 9.1 30 11,290,323 6,518,471 0.61 24.02.0 24 2 1.0 9.1 30 11,290,323 7,983,464 0.61 24.0 2.0 12 1 0.5 9.1 3011,290,323 11,290,323 0.61 24.0 2.0 6 0.5 0.3 9.1 30 11,290,32315,966,927 0.51 20.0 1.67 60.0 5.0 3.0 12.2 40 12,544,803 5,121,394 0.5321.0 1.75 63.0 5.3 3.0 12.2 40 13,172,043 5,377,464 0.56 22.0 1.83 66.05.5 3.0 12.2 40 13,799,283 5,633,534 0.58 23.0 1.92 69.0 5.8 3.0 12.2 4014,426,523 5,889,603 0.61 24.0 2.00 72.0 6.0 3.0 12.2 40 15,053,7636,145,673 0.64 25.0 2.08 75.0 6.3 3.0 12.2 40 15,681,004 6,401,743 0.6626.0 2.17 78.0 6.5 3.0 12.2 40 16,308,244 6,657,813 0.69 27.0 2.25 81.06.8 3.0 12.2 40 16,935,484 6,913,882 0.71 28.0 2.33 84.0 7.0 3.0 12.2 4017,562,724 7,169,952 0.74 29.0 2.42 87.0 7.3 3.0 12.2 40 18,189,9647,426,022 0.76 30.0 2.50 90.0 7.5 3.0 12.2 40 18,817,204 7,682,091 0.7630.0 2.50 60.0 5.00 2.00 15.2 50 23,521,505 11,760,753

Operation of the loop reactors 50A, 50B may include feeding the sameamount of monomer or comonomer and/or the same amount of hydrogen intoeach loop reactor. An alternative operation may include feeding agreater amount of a comonomer to the first polymerization reactor thanto the second polymerization reactor, or vice versa. The feeding stepcan be conducted concurrently or sequentially. The operation may alsoinclude feeding a greater amount of hydrogen to the secondpolymerization reactor than the first polymerization reactor, or viceversa. Further, the same or a different comonomer concentration may bemaintained in each reactor 50A, 50B. Likewise, the same or a differenthydrogen concentration may be maintained in each reactor 50A, 50B.Furthermore, the first polyolefin (i.e., polyolefin polymerized in thefirst reactor 50A) may have a first range of physical properties, andthe second polyolefin (i.e., polyolefin polymerized in the secondreactor 50B) may have a second range of physical properties. The firstrange and the second range of physical properties may be the same ordifferent. Exemplary physical properties include but are not limited topolyolefin density, comonomer percentage, short chain branching amount,molecular weight, viscosity, melt index, melt flow rate, crystallinity,and the like.

At step 430, the slurry is withdrawn from the loop reactor zone. In oneexample, the slurry is continuously withdrawn from one or morecontinuous take off assemblies extending from at least one of the elbowsections or the horizontal sections. In another example, the slurry isperiodically withdrawn from one or more settling leg assembliesextending from the horizontal section 220.

Moreover, a differing amount of polyolefin may be produced and withdrawnfrom the loop reactor 50A, 50B. For example, the first polyolefinproduct produced from the loop reactor 50A may be 30 weight % to 70weight % of the second polyolefin product produced from the loop reactorSOB, or vice versa. The different amount of polyolefin production in theloop reactors 50A, 50B may be accommodated and adjusted with differentprocess conditions and/or system configurations and so on.

At step 440, the diluents and the olefin monomers within the fluidslurry are separated from the polyolefin polymer particles. For example,the diluents and the olefin monomers may be separated and recoveredwithin the recovery system 24, as described below. At step 450, apolyolefin polymer is obtained from the polyolefin production system100, as described below.

Referring back to FIG. 1, the reactor system 20 having one or more loopreactors coupled thereto is connected to the recovery system 24 via thefluff slurry product line 22. The recovery system 24 is configured toreceive the fluid slurry discharged from the reactor system 20 andseparate the fluid slurry into a polyolefin fluff stream 28 andnon-polymer components. For example, the liquid in the fluid slurry canbe treated within the recovery system 24 to be partially or fullyvaporized in a flash line (not shown). The vapor of the non-polymercomponents (e.g., diluent and unreacted monomer) can be separate fromthe polyolefin fluff stream 28. Examples of non-polymer componentspresent in the polyolefin fluff may includes diluents, unreactedmonomer/comonomer, and residual catalysts.

A flash line with a flash line heater coupled thereto can be used tovaporize and volatize diluents and increase enthalpy of the fluidslurry. The flash line and the flash line heater may be configured aspart of the reactor system, part of the recovery system 24, oralternatively, disposed between the reactor system 20 and the recoverysystem 24.

The non-polymer components can be recovered from a vapor phase into aliquid phase within the recovery system 24. In addition, the recoverysystem 24 may be configured to remove undesirable heavy and light chainsfrom the non-polymer components. For example, olefin-free diluent may berecovered from the recovery system 24 and reused, by delivering to thefeed system 16 and/or the reactor system 20.

The non-polymer components may be delivered through one or morenon-polymer flow-lines 26 to the fractionation system 30 to befractionated and/or treated into fractionations of recovered non-polymerdiluent, monomer, and/or catalyst components. The fractionations ofnon-polymer components are thus recovered and reused by delivering thefractions into the reactor system 20, directly or via a fractionationfeed line 32, which is connected to the feed system 16. A by-pass linefeed 34 can be used to deliver the non-polymer components from recoversystem 24 (e.g., via the non-polymer flow-line 26) to the feed line 16(e.g., via the fractionation feed line 32) by bypassing thefractionation system 30.

The polyolefin fluff stream 28 discharged from the recovery system 24 isthen delivered to the extrusion system 36 and extruded into a polyolefinpellet 38 with desired mechanical, physical, and melt characteristics.The extrusion system 36 may include an extruder (e.g., a pelletizer),which is configured to add additives to a feed of the polyolefin fluffstream 28 to impart desired characteristics of the polyolefin pellets 38finally obtained. The extruder heats and melts the feed of thepolyolefin fluff stream 28, and extrudes the feed (e.g., via a twinscrew extruder) through a pelletizer die under pressure to obtain thepolyolefin pellets 38. Such pellets can be cooled in a water systemdisposed at or near the discharge of the pelletizer. Suitable additivesintroduced to the feed of the polyolefin fluff stream 28 to form thepolyolefin pellets 38 may include surface modifiers (e.g., slip agents,antiblocks, tackifers), UV inhibitors, antioxidants (e.g., phenolics,phosphites, thioesters, amines, etc.), colorants, pigments, processingaids (e.g., flow promoters such as waxes & oils and fluoroeslastomers),peroxides, and other additives. Different additives may be combined intodifferent additive packages to be dispensed into one or more extruderfeed tanks and extruders for obtaining polyolefin pellets 38 ofdifferent desired characteristics.

As shown in FIG. 1, the dry finish end 44 of the polyolefin productionsystem 100 also includes the loadout system 39 configured to prepare thepolyolefin pellets 38 for shipment to customers 40. In general, thepolyolefin pellets 38 may be transported to a loadout area to be stored,blended with other pellets, and/or loaded into railcars, trucks, bags,and so forth. However, the polyolefin pellets 38 are generally notaltered by the loadout system 39 prior to being sent to the customer 40.

Polyolefin pellets 38 generated from the polyolefin production system100 may include low density polyethylene (LDPE), linear low densitypolyethylene (LLDPE), medium density polyethylene (MDPE), high densitypolyethylene (HDPE), enhanced polyethylene, isotactic polypropylene(iPP), syndiotactic polypropylene (sPP), including various copolymers,and so on. The polyolefin pellets 38 may be used in the manufacturing ofa variety of products, components, household items and other items,including but not limited to adhesives (e.g., hot-melt adhesiveapplications), electrical wire and cable, agricultural films, shrinkfilm, stretch film, food packaging films, flexible food packaging, milkcontainers, frozen-food packaging, trash and can liners, grocery bags,heavy-duty sacks, plastic bottles, safety equipment, carpeting,coatings, toys and an array of containers and plastic products.

Specific types of polyolefins, such as high-density polyethylene (HDPE),have particular applications in the manufacture of blow-molded andinjection-molded goods, such as food and beverage containers, film, andplastic pipe. Other types of polyolefins, such as low-densitypolyethylene (LDPE), linear low-density polyethylene (LLDPE), isotacticpolypropylene (iPP), and syndiotactic polypropylene (sPP) are alsosuited for similar applications. The mechanical requirements of theapplication, such as tensile strength and density, and/or the chemicalrequirements, such thermal stability, molecular weight, and chemicalreactivity, typically determine what type ofpolyolefin is suitable.

To manufacture the end-products, the polyolefin pellets 38 are generallysubjected to processing, such as blow molding, injection molding,rotational molding, blown film, cast film, extrusion (e.g., sheetextrusion, pipe and corrugated extrusion, coating/lamination extrusion,etc.), and so on. Ultimately, the products and components made from thepolyolefin pellets 38 may be further processed and assembled fordistribution and sold to consumers. For example, extruded pipe or filmmay be packaged for distribution to the customer, or a fuel tankcomprising polyethylene may be assembled into an automobile fordistribution and sold to consumers.

Process variables and parameters in the polyolefin production system 100may be controlled automatically and/or manually via various valveconfigurations, control systems, and so on. In general, aprocessor-based control system, such as the control system 46 as shownin FIG. 1, may facilitate management of a range of operations in thepolyolefin production system 100. A polyolefin manufacturing facilitymay include a central control room, as well as a central control system,such as a distributed control system (DCS) and/or programmable logiccontroller (PLC). For example, the reactor system 20 may employ aprocessor-based system, such as a DCS, or other advanced process controlsystems known in the art. The control system 46 may include one or moreDCS to control the feed system 16, the reactor system 20, the recoverysystem 24, and/or the fractionation system 30. In the dry end 44 of apolyolefin production plant, the extrusion system 36 and/or the pelletloadout system 39 may also be controlled via a processor-based system(e.g., DCS or PLC). Moreover, computer-readable media may store controlsystem executable codes to be executed by associated processorsincluding central processing units, and the like. The computer readablemedium can refer to any storage medium that may be used to inconjunction with computer readable instructions. In an exemplary andnon-limiting illustrative embodiment, the computer readable medium caninclude a computer readable storage medium. The computer readablestorage medium can take many forms, including, but not limited to,non-volatile media and volatile media, floppy disks, flexible disks,hard disks, magnetic tape, other magnetic media, CD-ROMs, DVDs, or anyother optical storage medium, punch cards, paper tape, or any otherphysical medium with patterns of holes. Computer readable storage mediacan further include RAM, PROM, EPROM, EEPROM, FLASH, combinationsthereof (e.g., PROM EPROM), or any other memory chip or cartridge. Thecomputer readable medium can further include computer readabletransmission media. Such transmission media can include coaxial cables,copper wire and fiber optics. Transmission media may also take the formof acoustic or light waves, such as those generated during radiofrequency, infrared, wireless, or other media comprising electric,magnetic, or electromagnetic waves.

Accordingly, the DCS and associated control system(s) in the polyolefinproduction system 100 may include appropriate hardware, software logicand codes, to interface with the various process equipment, controlvalves, conduits, and instrumentation, to facilitate the measurement andcontrol of process variables, to implement control schemes, to performcalculations, and so on. A variety of instrumentation known to those ofordinary skill in the art may be provided to measure process variables,such as pressure, temperature, flow rate, and so on, and to transmit asignal to the control system 46, where the measured data may be read byan operator and/or used as an input in various control functions.Depending on the application and other factors, indication of theprocess variables may be read locally or remotely by an operator, andused for a variety of control purposes via the control system.

A plant manager, engineer, technician, supervisor and/or operator canmonitor and control the process in the control room. When using a DCS,the control room may be the center of activity, facilitating theeffective monitoring and control of the process or facility. The controlroom and DCS may contain a Human Machine Interface (HMI), which may be acomputer, with specialized software to provide a user-interface for thecontrol system. The HMI may vary by vendor and present the user with agraphical version of the manufacturing process conducted within thepolyolefin production system 100. There may be multiple HMI consoles orworkstations, with varying degrees of access to data.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered,combined, or modified and all such variations are considered within thescope and spirit of the present invention.

The invention illustratively disclosed herein suitably may be practicedin the absence of any element that is not specifically disclosed hereinand/or any optional element disclosed herein. While compositions andmethods are described in terms of “comprising,” “containing,” or“including” various components or steps, the compositions and methodscan also “consist essentially of” or “consist of” the various componentsand steps. All numbers and ranges disclosed above may vary by someamount. Whenever a numerical range with a lower limit and an upper limitis disclosed, any number and any included range falling within the rangeare specifically disclosed. In particular, every range of values (of theform, “from about a to about b,” or, equivalently, “from approximately ato b,” or, equivalently, “from approximately a-b”) disclosed herein isto be understood to set forth every number and range encompassed withinthe broader range of values.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

What is claimed is:
 1. A loop reactor for olefin polymerization, theloop reactor comprising: a plurality of vertical sections; a pluralityof elbow sections connecting the vertical sections to either ahorizontal section having a horizontal length (L_(H)) or another elbowsection, at least one elbow section having an internal diameter (d), aradius (R_(c)) of an inner curvature and a chord length (W) and whereinthe horizontal length (L_(H)) is from 0 feet to 3 feet, the chord length(W) is 250 inches or less and a ratio (R_(c)/d) of the radius (R_(c)) ofthe inner curvature to the internal diameter (d) of the at least oneelbow section is maintained from 2 to 4; and at least one loop reactionzone configured to polymerize an olefin monomer in the presence of aliquid diluent into a slurry comprising particles of a polyolefinpolymer.
 2. The loop reactor of claim 1, wherein the horizontal length(L_(H)) is from 0.5 feet to 2.0 feet.
 3. The loop reactor of claim 1,wherein the at least one elbow section connects a vertical sectionhaving a vertical length (L_(V)) to the horizontal section and where thevertical length (LV) is at least 60 times longer than the horizontallength (L_(H)).
 4. The loop reactor of claim 1, wherein the at least oneelbow section connects a vertical section to another elbow section. 5.The loop reactor of claim 1, further comprises one or more settling legassemblies extending from the horizontal section, one or more continuoustake off (CTO) assemblies extending from at least one of the elbowsections or the horizontal section or a combination thereof.
 6. The loopreactor of claim 1, wherein the at least one elbow section comprises aheight (H) and wherein the radius (R_(c)) of the inner curvature,measured as R_(c)=H/2+W²/8H, is 72 inches or less.
 7. The loop reactorof claim 1, wherein the circulation velocity (V) of the slurry withinthe at least one loop reaction zone is maintained at 9 meters per secondor higher.
 8. The loop reactor of claim 1, wherein a Reynolds number ofthe slurry within the at least one elbow section is maintained at11,000,000 or higher.
 9. The loop reactor of claim 1, wherein two loopreaction zones are formed, each loop reaction zone is formed by fourvertical sections, four horizontal sections, and eight elbow sectionsand wherein the two loop reaction zones are connected to continuouslytransfer the slurry contained therein.